U.S. patent number 7,862,935 [Application Number 11/130,807] was granted by the patent office on 2011-01-04 for management via dynamic water holdup estimator in a fuel cell.
This patent grant is currently assigned to GM Global Technology Operations, Inc.. Invention is credited to John C. Fagley, Steven G. Goebel, Manish Sinha, Peter Willimowski.
United States Patent |
7,862,935 |
Sinha , et al. |
January 4, 2011 |
Management via dynamic water holdup estimator in a fuel cell
Abstract
A strategy of controlling a state of hydration of a fuel cell(s)
and actively managing operation of the fuel cell(s) to achieve a
desired state of hydration. The control strategy monitors the state
of hydration and a rate of change of the state of hydration which
are used to control the operation of the fuel cell(s). A
supervisory control strategy is implemented that alters the
operating parameters of the fuel cell(s) based upon the state of
hydration, the rate of change of the state of hydration, and a
desired operational range for the state of hydration.
Inventors: |
Sinha; Manish (Pittsford,
NY), Fagley; John C. (Victor, NY), Willimowski; Peter
(Rossdorf, DE), Goebel; Steven G. (Victor, NY) |
Assignee: |
GM Global Technology Operations,
Inc. (Detroit, MI)
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Family
ID: |
37387872 |
Appl.
No.: |
11/130,807 |
Filed: |
May 17, 2005 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20060263653 A1 |
Nov 23, 2006 |
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Current U.S.
Class: |
429/413; 429/400;
429/450; 429/414 |
Current CPC
Class: |
H01M
8/04455 (20130101); H01M 8/04126 (20130101); H01M
8/04768 (20130101); H01M 8/2457 (20160201); H01M
8/04753 (20130101); H01M 8/0258 (20130101); H01M
8/04302 (20160201); H01M 8/04507 (20130101); H01M
8/04723 (20130101); H01M 8/0485 (20130101); H01M
8/0267 (20130101); H01M 8/04335 (20130101); H01M
8/04358 (20130101); H01M 8/04223 (20130101); H01M
8/241 (20130101); H01M 8/04992 (20130101); H01M
8/04529 (20130101); H01M 8/04395 (20130101); H01M
8/04835 (20130101); H01M 8/04708 (20130101); H01M
8/04559 (20130101); Y02E 60/50 (20130101); H01M
8/04589 (20130101) |
Current International
Class: |
H01M
8/04 (20060101); H01M 8/00 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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6-245158 |
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Sep 1994 |
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JP |
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9-128564 |
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May 1997 |
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JP |
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Other References
US. Appl. No. 11/130,804, filed May 17, 2005, Fagley et al.,
"Relative Humidity, Profile Control Strategy for High Current
Density Stack Operation". cited by other .
U.S. Appl. No. 11/130,806, filed May 17, 2005,Victor W. Logan,
"Fuel Cell System Relative Humidity". cited by other .
U.S. Appl. No. 11/130,825, filed May 17, 2005, Goebel et al.,
"Relative Humidity Control for a Fuel Cell". cited by other .
Fagley, John; Gu, Wenbin; and Whitehead, Lee, "Thermal Modeling of
a PEM Fuel Cell," pp. 141-149, published by ASME in "Fuel Cell
Science, Engineering and Technology," June 2004. cited by
other.
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Primary Examiner: Yuan; Dah-Wei
Assistant Examiner: Martin; Angela
Claims
What is claimed is:
1. A method of operating a fuel cell comprising: (a) monitoring a
state of hydration of the fuel cell, including monitoring a rate of
change of said state of hydration; and (b) adjusting an operating
parameter of the fuel cell based upon said state of hydration.
2. The method of claim 1, wherein (b) includes adjusting an
operating parameter of the fuel cell based upon said state of
hydration and said rate of change of said state of hydration.
3. The method of claim 2, wherein monitoring a rate of change of
said state of hydration includes calculating a rate of change of
said state of hydration based upon an equation that predicts said
rate of change of said state of hydration.
4. The method of claim 3, wherein calculating said rate of change
of said state of hydration includes accounting for water entering
the fuel cell, water generated in the fuel cell, and water leaving
the fuel cell.
5. The method of claim 2, wherein (a) includes estimating a state
of hydration of the fuel cell at a first time t.sub.1 by taking a
previously estimated rate of change of said state of hydration at a
second time t.sub.2 multiplied by a time difference between t.sub.1
and t.sub.2 and adding a previously estimated state of hydration at
said second time t.sub.2.
6. The method of claim 1, further comprising comparing said state
of hydration to a predetermined standard or range and wherein (b)
includes adjusting said operating parameter based upon said
comparison.
7. The method of claim 6, further comprising adjusting said
predetermined standard or range based upon a previous operating
performance of the fuel cell.
8. The method of claim 7, wherein adjusting said predetermined
standard or range includes comparing an instantaneous voltage and
electrical current generation of the fuel cell to a previously
achieved optimal voltage and electrical current generation of the
fuel cell.
9. The method of claim 1, wherein (a) includes determining an
initial state of hydration of the fuel cell at start up based upon
a high frequency resistance of the fuel cell.
10. The method of claim 1, wherein (a) includes determining an
initial state of hydration of the fuel cell at start up based upon
an ending state of hydration of the fuel cell at a previous
shutdown of operation of the fuel cell.
11. A method of operating a fuel cell comprising: (a) monitoring a
state of hydration of the fuel cell including estimating an initial
state of hydration of the fuel cell as 100%; (b) adjusting an
operating parameter of the fuel cell based upon said state of
hydration; starting up operation of the fuel cell from a cold
condition; and allowing the fuel cell to flood following said cold
start up.
12. The method of claim 1, further comprising: determining a high
frequency resistance of the fuel cell; ascertaining a state of
hydration of the fuel cell based upon said high frequency
resistance; comparing said monitored state of hydration with said
ascertained state of hydration based upon said high frequency
resistance; and adjusting said state of hydration based upon said
comparison.
13. The method of claim 1, wherein (b) includes determining a
target cathode effluent relative humidity based upon said state of
hydration and adjusting an operating parameter based on said target
cathode effluent relative humidity.
14. The method of claim 1, wherein (b) includes using an algorithm
and said state of hydration to determine an adjustment to an
operating parameter of the fuel cell.
15. The method of claim 1, wherein (b) includes using a look-up
table and said state of hydration to determine an adjustment to an
operating parameter of the fuel cell.
16. The method of claim 1, wherein (b) includes adjusting one or
more of a relative humidity of a cathode reactant flowing into the
fuel cell, a coolant temperature flowing through the fuel cell, a
pressure of a cathode reactant in the fuel cell, and stoichiometric
quantity of cathode reactant flowing into the fuel cell.
17. The method of claim 1, wherein (b) includes adjusting an
operating parameter of the fuel cell to maintain said state of
hydration in a predetermined range.
18. A method of operating a fuel cell stack comprising: (a)
monitoring a state of hydration of the fuel cell stack including
ascertaining a current state of hydration of the fuel cell stack
and ascertaining a current rate of change of said state of
hydration of the fuel cell stack; (b) adjusting an operating
parameter of the fuel cell stack based upon said state of
hydration, including adjusting an operating parameter of the fuel
cell stack based upon said current state of hydration and said
current rate of change of said state of hydration and said current
rate of change of said state of hydration; and allowing the fuel
cell stack to flood following said start up.
19. The method of claim 18, wherein (a) includes ascertaining a
current state of hydration based upon a previously ascertained
state of hydration and a previously ascertained rate of change of
said state of hydration.
20. The method of claim 19, wherein (a) includes ascertaining a
current state of hydration at a first time t.sub.1 by taking a
previously ascertained rate of change of said state of hydration at
a second time t.sub.2 multiplied by a time difference between
t.sub.1 and t.sub.2 and adding a previously ascertained state of
hydration at said second time t.sub.2.
21. The method of claim 18, further comprising comparing said
current state of hydration to a predetermined standard or range and
wherein (b) includes adjusting said operating parameter based upon
said comparison.
22. The method of claim 21, further comprising adjusting said
predetermined standard or range based upon a previous operating
performance of the fuel cell stack.
23. The method of claim 18, wherein (a) includes ascertaining an
initial state of hydration of the fuel cell stack at start up based
upon a high frequency resistance of the fuel cell stack.
24. The method of claim 18, wherein (a) includes ascertaining an
initial state of hydration of the fuel cell stack at start up based
upon an ending state of hydration of the fuel cell stack at a
previous shutdown of operation of the fuel cell stack.
25. The method of claim 18, further comprising: starting up
operation of the fuel cell stack; and wherein (a) includes
estimating an initial state of hydration of the fuel cell stack as
100%.
26. The method of claim 18, further comprising: ascertaining a high
frequency resistance of one or more fuel cells in the fuel cell
stack; and adjusting said ascertained current state of hydration
based upon said high frequency resistance.
27. The method of claim 18, wherein (b) includes determining a
target cathode effluent relative humidity based upon said current
state of hydration and said current rate of change of said state of
hydration and adjusting an operating parameter of the fuel cell
stack based on said target cathode effluent relative humidity.
28. The method of claim 18, wherein (b) includes using an algorithm
and said current state of hydration and said current rate of change
of said state of hydration to determine an adjustment to an
operating parameter of the fuel cell stack.
29. The method of claim 18, wherein (b) includes using a look-up
table and said current state of hydration and said current rate of
change of said state of hydration to determine an adjustment to an
operating parameter of the fuel cell stack.
30. The method of claim 18, wherein (b) includes adjusting one or
more of a relative humidity of a cathode reactant flowing into the
fuel cell stack, a coolant temperature flowing through the fuel
cell stack, a pressure of a cathode reactant in the fuel cell
stack, and a stoichiometric quantity of cathode reactant flowing
into the fuel cell stack.
Description
FIELD OF THE INVENTION
The present invention relates to fuel cells and, more particularly
to controlling the operation of fuel cells based upon a state of
hydration of the fuel cells.
BACKGROUND OF THE INVENTION
Fuel cells are used as a power source for electric vehicles,
stationary power supplies and other applications. One known fuel
cell is the PEM (i.e., Proton Exchange Membrane) fuel cell that
includes a so-called MEA ("membrane-electrode-assembly") comprising
a thin, solid polymer membrane-electrolyte having an anode on one
face and a cathode on the opposite face. The MEA is sandwiched
between a pair of electrically conductive contact elements which
serve as current collectors for the anode and cathode, which may
contain appropriate channels and openings therein for distributing
the fuel cell's gaseous reactants (i.e., H.sub.2 and O.sub.2/air)
over the surfaces of the respective anode and cathode.
PEM fuel cells comprise a plurality of the MEAs stacked together in
electrical series while being separated one from the next by an
impermeable, electrically conductive contact element known as a
bipolar plate or current collector. In some types of fuel cells
each bipolar plate is comprised of two separate plates that are
attached together with a fluid passageway therebetween through
which a coolant fluid flows to remove heat from both sides of the
MEAs. In other types of fuel cells the bipolar plates include both
single plates and attached together plates which are arranged in a
repeating pattern with at least one surface of each MEA being
cooled by a coolant fluid flowing through the two plate bipolar
plates.
The fuel cells are operated in a manner that maintains the MEAs in
a humidified state. The level of humidity or hydration of the MEAs
affects the performance of the fuel cell. Too wet of an MEA limits
the performance of the fuel cell stack. Specifically, formation of
liquid water impedes the diffusion of gas to the MEAs, thereby
limiting their performance. The liquid water also acts as a flow
blockage reducing cell flow and causing even higher fuel cell
relative humidity which can lead to unstable fuel cell performance.
Additionally, the formation of liquid water within the cell may
cause significant damage when the fuel cell is shut down and
exposed to freezing conditions. That is, when the fuel cell is
nonoperational and the temperature in the fuel cell drops below
freezing, the liquid water therein will freeze and expand,
potentially damaging the fuel cell. Too dry of an MEA also limits
the performance. Specifically, as the humidity level decreases the
protonic resistance of the MEA will start to increase (especially
near the inlet), resulting in additional waste heat and lower
production of electricity. Furthermore, durability data suggests
that large cycling in the moisture content of the MEA that leads to
repeated flooding and drying of membranes can lead to significant
loss in durability due to membrane swelling and shrinking. Thus,
repeated flooded and dry operating conditions lead to a loss of
overall efficiency and may reduce the durability of the MEA and the
fuel cell.
Accordingly, it is advantageous to control the operation of the
fuel cell in a manner that allows for efficient operation of the
fuel cell and/or minimizes an impact on the durability of the MEA
and fuel cell. Prior control strategies to manage the operation of
the fuel cell have focused on maintaining a cathode effluent
relative humidity at a constant level. Such strategies, however, do
not monitor the state of hydration of the fuel cell and/or fuel
cell stack (i.e., how much water buffer is in the membrane,
diffusion media and channels). Additionally, the prior control
strategies do not actively manage process excursions that may lead
to drying and flooding of the fuel cell and/or fuel cell stack.
SUMMARY OF THE INVENTION
The present invention monitors a state of hydration of the fuel
cell and/or fuel cell stack and controls operation of the fuel cell
and/or fuel cell stack based upon the state of hydration. The rate
of change of the state of hydration is also monitored and used to
control the operation of the fuel cell and/or fuel cell stack. A
supervisory control strategy is implemented that alters the
operating parameters of the fuel cell and/or fuel cell stack based
upon the state of hydration, the rate of change of the state of
hydration and a desired operational range for the state of
hydration. Accordingly, the control strategy of the present
invention actively manages process excursions that may lead to
drying and flooding of the fuel cell and/or fuel cell stack.
A method of operating a fuel cell according to the principle of the
present invention includes: (1) monitoring a state of hydration of
the fuel cell; and (2) adjusting an operating parameter of the fuel
cell based upon the state of hydration.
In another aspect of the present invention, a method of operating a
fuel cell system having a fuel cell stack is disclosed. The method
includes: (1) ascertaining a current state of hydration of the fuel
cell stack; (2) ascertaining a current rate of change of the state
of hydration of the fuel cell stack; and (3) adjusting an operating
parameter of the fuel cell stack based upon the current state of
hydration and the current rate of change of the state of
hydration.
In yet another aspect of the present invention, a method of
operating a fuel cell system having a fuel cell stack operable to
convert a cathode reactant flowing through a cathode flow path and
an anode reactant flowing through an anode flow path into
electrical energy is disclosed. The method includes: (1)
ascertaining a current state of hydration of the fuel cell stack;
(2) ascertaining a current rate of change of the state of hydration
of the fuel cell stack; (3) comparing the current state of
hydration to a predetermined standard or range; (4) determining a
target cathode effluent relative humidity based on the comparison;
and (5) adjusting an operating parameter of the fuel cell stack
based on the target cathode effluent relative humidity.
Further areas of applicability of the present invention will become
apparent from the detailed description provided hereinafter. It
should be understood that the detailed description and specific
examples, while indicating the preferred embodiment of the
invention, are intended for purposes of illustration only and are
not intended to limit the scope of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will become more fully understood from the
detailed description and the accompanying drawings, wherein:
FIG. 1 is a schematic representation of an exemplary fuel cell
system in which the control strategy of the present invention can
be utilized;
FIG. 2 is a schematic representation of the supervisory approach
for managing the state of hydration of a fuel cell stack according
to the principle of the present invention; and
FIG. 3 is a flow chart illustrating the control strategy of the
present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The following description of the preferred embodiment is merely
exemplary in nature and is in no way intended to limit the
invention, its application, or uses.
An exemplary fuel cell system 20 in which the control strategy
according to the principle of the present invention can be used is
illustrated in FIG. 1. Fuel cell system 20 includes a fuel cell
stack 22 which is comprised of a plurality of fuel cells 24
arranged adjacent one another to form stack 22. Fuel cells 24
include membrane-electrode-assemblies (MEAs) separated from each
other by electrically conductive, liquid-cooled bipolar separator
plates. The fuel cells 24 that are on the ends of stack 22 are
disposed between terminal plates and end contact fluid distribution
elements. The end fluid distribution elements as well as the
working faces or sides of each bipolar plate contain a plurality of
lands adjacent to grooves or channels on the active faces and form
flow fields (flow paths) for distributing anode and cathode
reactants (i.e., H.sub.2 and O.sub.2/air) to the MEAs.
Gas-permeable conductive diffusion media press up against the
electrode faces of the MEAs and between end contact fluid
distribution elements and terminal collector plates to provide a
conductive pathway therebetween when the stack is compressed during
normal operating conditions.
Cathode reactant, in this case in the form of air, is supplied to
the cathode flow field of fuel cell stack 22 via compressor 26 and
cathode supply plumbing 28. Alternatively, the cathode reactant can
be supplied from a pressurized storage tank (not shown). The
cathode reactant gas flows from compressor 26 through a humidifying
device 30, in this case in the form of a water vapor transfer (WVT)
device wherein the cathode reactant gas is humidified. The cathode
reactant gas then flows through an optional heat exchanger 32
wherein the cathode reactant gas can be heated or cooled, as
needed, prior to entering fuel cell stack 22.
The cathode reactant gas then flows through the cathode reactant
flow fields (cathode flow path) of fuel cell stack 22 and exits
fuel cell stack 22 in the form of cathode effluent via cathode
exhaust plumbing 34. The cathode effluent is routed through WVT
device 30.
Within WVT device 30, humidity from the cathode effluent stream is
transferred to the cathode reactant gas being supplied to fuel cell
stack 22. The operation of WVT device 30 can be adjusted to provide
differing levels of water vapor transfer between the cathode
effluent stream and the cathode reactant stream.
Anode reactant in the form of H.sub.2 is supplied to the anode flow
fields (anode flow path) of fuel cell stack 22 via anode supply
plumbing 36. Anode reactant gas can be supplied from a storage
tank, a methanol or gasoline reformer, or the like. The anode
reactant flows through the anode reactant flow path and exits fuel
cell stack 22 in the form of anode effluent via anode exhaust
plumbing 38.
Coolant is supplied to the coolant flow path within fuel cell stack
22 via coolant supply plumbing 40 and is removed from fuel cell
stack 22 via coolant exit plumbing 42. The coolant flowing through
fuel cell stack 22 removes heat generated therein by the reaction
between the anode and cathode reactants. The coolant can also
control the temperature of the cathode reactant and/or cathode
effluent as it travels throughout the cathode reactant flow path
within fuel cell stack 22. Optionally, the coolant can flow through
heat exchanger 32 prior to entering fuel cell stack 22 thereby
equalizing the temperature of the cathode reactant gas and the
coolant prior to entering fuel cell stack 22. In this manner, the
temperature of the cathode reactant flowing into the fuel cell
stack 22 can be controlled to a desired set point. The coolant and
cathode reactant equalize to a same temperature very quickly within
fuel cell stack 22. Accordingly, the temperature of the cathode gas
is substantially the same as the temperature of the coolant as the
flows progress through fuel cell stack 22.
A controller 46 communicates with the various components of fuel
cell system 20 to control and coordinate their operation. For
example, controller 46 communicates with compressor 26 to control
the stoichiometric quantity of cathode reactant supplied to fuel
cell stack 22. Controller 46 also communicates with WVT device 30
to control the humidification of the cathode reactant flowing into
fuel cell stack 22. Controller 46 communicates with heat exchanger
32 to control the temperature of the cathode reactant flowing into
fuel cell stack 22. Controller 46 also communicates with the
coolant supply system to control the flow rate of coolant through
fuel cell stack 22 and also the temperature of the coolant routed
through fuel cell stack 22. Controller 46 also communicates with
the anode reactant supply system to control the quantity of anode
reactant supplied to fuel cell stack 22.
Controller 46 may be a single controller or multiple controllers
whose actions are coordinated to provide a desired overall
operation of fuel cell system 20. Furthermore, controller 46 may
include one or more modules, as needed, to perform the
functionality indicated. As used herein, the term "module" refers
to an application specific integrated circuit (ASIC), an electronic
circuit, a processor (shared, dedicated or group) and memory that
execute one or more software or firmware programs, a combinational
logic circuit, or other suitable components that provide the
desired functionality.
The present invention provides a strategy for controlling the state
of hydration or moisture content of the membranes and other soft
goods of fuel cells 24 within fuel cell stack 22. The desired
operating conditions of fuel cell stack 22 and fuel cell system 20
are typically defined in terms of intervals of process conditions,
such as pressure, temperature, stoichiometry and relative humidity
within the stack. The resulting multi-variable space (operating
condition space or OCS) defines the steady state normal operating
boundary that results in best performance and durability of fuel
cell stack 22. Transient operation often results in stack
conditions outside the OCS resulting in drying or wetting of the
stack, the membrane and the soft goods. For example, the
temperature falling below the OCS boundary causes reduction in
water carrying capacity of cathode effluent and eventually results
in water accumulation within fuel cell stack 22 leading to stack
flooding. Similarly, temperature rising above the OCS boundary
causes more water to be removed via cathode effluent than is being
generated by reaction and leads to membrane drying. Similar
excursions in pressure, stoichiometry and inlet relative humidity
of the cathode reactant or any combination may also lead to
flooding or drying of fuel cell stack 22. As stated above, both
flooding and drying conditions may lead to a loss of performance of
fuel cell stack 22. Moreover, cycling between these conditions has
been shown to have a detrimental effect on the durability of fuel
cell stack 22 due to membrane swelling and shrinking.
Excursions outside the OCS boundary are expected to happen in a
real system due to dynamic limitations of components in following
the load profile in a typical drive cycle. The loss of performance
and durability are not directly due to excursions in process
conditions but rather the effect that excursions in process
conditions have on the water accumulation or holdup within the
stack. To address this, the present invention uses a supervisory
control strategy that monitors the state of hydration (SOH) of fuel
cell stack 22 and manages the desired set point for the stack
relative humidity to maintain the water holdup within the membranes
of fuel cell stack 22 within an optimal range.
Referring now to FIG. 2, a schematic representation of the
supervisory loop employed by controller 46 to control the operation
of fuel cell stack 22 and fuel cell system 20 according to the
methods of the present invention is shown. The operational
parameters of fuel cell stack 22 are monitored by various sensors
and the output from those sensors is communicated to a holdup
observer module 50. For example, the inlet and outlet pressures of
the cathode gas flowing into/out of the cathode flow path, the
inlet and outlet coolant temperatures, the stoichiometric quantity
(ST) of cathode and anode reactant flowing into fuel cell stack 22,
and the relative humidity of the cathode and anode reactant flowing
into fuel cell stack 22 are communicated to holdup observer module
50. Holdup observer module 50 uses an estimator for the state of
hydration and also determines the rate of change of the state of
hydration which is used to adjust or control the operation of fuel
cell stack 22 to provide the desired performance. Based on the
state of hydration and the rate of change, holdup observer module
50 determines a target set point for the cathode effluent relative
humidity, as described below.
The fuel cell state of hydration (SOH) is a measure of total water
uptake (M.sub.w) in the fuel cell which includes water in the
membrane and the diffusion media. Membrane hydration is defined as
the ratio of water molecules per sulfonic group (.lamda.) in a
nafion membrane used in a PEM fuel cell. Variations in .lamda.
affects the membrane's proton conductivity and can be measured via
a high frequency resistance (HFR) of fuel cell stack 22. The extent
of water holdup in diffusion media (DM) is defined as .theta.. The
parameter .theta. is a measure of the amount of water in the DM
relative to the maximum amount of water uptake in the DM, and takes
value between 0 and 1. The water holdup in diffusion media starts
only after the membrane is complete saturated, i.e.
.lamda.=.lamda..sub.max, where .lamda..sub.max is determined by the
material property (such as density of sulphonic acids) of the
membrane. In other words .theta. is greater than 0 only when
.lamda.=.lamda..sub.max and .lamda. is less than .lamda..sub.max
only when .theta.=0. Thus, variation in partial hydration of the
membrane can be detected and estimated via HFR of fuel cell stack
22. However, when the membrane is flooded with water and the water
accumulation is occurring in the diffusion media and/or channels of
the fuel cells 24, the HFR signal is not able to measure variations
in .lamda.. In other words, the HFR signal is no longer indicative
of changes occurring in .lamda. once the membrane has been flooded.
Accordingly, the HFR measurement may be used during certain
operational conditions to determine a state of hydration or a
measure of .lamda. which can be used to control the operation of
fuel cell stack 22 and fuel cell system 20, as described in more
detail below. The measurement of HFR is explained in detail in U.S.
Pat. No. 6,376,111 entitled "System and Method for Controlling the
Humidity Level of a Fuel Cell," the disclosure of which is
incorporated herein by reference.
Holdup observer module 50 is operable to estimate a state of
hydration of the fuel cell membrane and to estimate a rate of
change of the state of hydration as a function of time. The
estimated state of hydration can be determined in a number of ways,
as described below. The estimated rate of change of the state of
hydration is calculated using a formula that takes into account the
water flowing in, the water generated within, and the water flowing
out of fuel cell stack 22, as described in more detail below. Based
upon the estimated state of hydration and the estimated rate of
change of the state of hydration, operation of fuel cell stack 22
is adjusted to provide an optimal or desired operating
performance.
During operation of fuel cell stack 22, an initial estimate or
determination of the state of hydration of the membranes within
fuel cell stack 22 is needed. This initial state of hydration can
be determined in a number of manners. A first method of determining
the initial state of hydration is based upon a previous operation
of fuel cell stack 22. Specifically, during a previous operation of
fuel cell stack 22, the shutdown procedure utilized to cease the
operation of fuel cell stack 22 is controlled to provide a desired
ending state of hydration for the fuel cell stack. This ending
state of hydration is then used as the initial state of hydration
for fuel cell stack 22 upon a subsequent startup of operation. The
state of hydration of fuel cell stack 22 should not change during
the nonuse period because there is no gas flow through the anode
and cathode flow fields.
A second way of determining the initial state of hydration is by
measuring the HFR of fuel cell stack 22 initially upon startup. The
measure of HFR is indicative of the state of hydration and, thus,
yields an initial state of hydration that can be utilized when
starting the fuel cell stack. It should be appreciated, however,
that this method is only applicable when fuel cell stack 22 is not
in a flooded condition. Accordingly, if fuel cell stack initially
begins operation in a flooded condition, the use of an HFR
measurement to determine the initial state of hydration is not
feasible.
A third way of determining the initial state of hydration is to
allow operation of the fuel cell stack 22 to begin under conditions
that would lead to flooding of the stack after a short period of
time (e.g., 5-10 seconds). The initial state of hydration is
assumed to be 100% which corresponds to a flooded condition. The
use of such an assumption will not be too far off from an actual
measure of the state of hydration and, accordingly, can be used to
assume the initial state of hydration. When this method is
employed, continued operation of fuel cell stack 22 will typically
reduce the state of hydration of fuel cell stack 22 and will depart
from the flooded condition.
Thus, holdup observer module 50 is operable to determine an initial
state of hydration of fuel cell stack 22 through either a previous
state of hydration at shut down, a measure of HFR, or by assuming a
flooded condition and an initial SOH of 100%.
With the initial state of hydration ascertained, holdup observer
module 50 can continue to monitor the state of hydration of fuel
cell stack 22 during operation. Specifically, holdup observer
module 50 calculates a rate of change of the state of hydration and
uses that rate of change to dynamically adjust the state of
hydration of fuel cell stack 22, as described below. The rate of
change of the state of hydration can be calculated using the
following equation:
dd.times..times..times. ##EQU00001##
where:
M.sub.w=holdup of water in the fuel cell [moles/cell];
X.sub.in.sup.w=mole fraction of water in the inlet cathode reactant
stream (function of inlet RH);
n.sub.in.sup.a=molar flow rate of cathode reactant on a dry basis
at the inlet [moles/cell];
jA=current density*area of fuel cell (total current) [Amps];
F=Faradays constant [mole e.sup.-/Amp];
X.sub.out.sup.w=mole fraction of water in the cathode effluent
(function of outlet RH); and
n.sub.out.sup.a=molar flow rate of cathode effluent on a dry basis
[mole/sec].
The first term of equation (1) on the right hand side captures the
water that is entering fuel cell stack 22 with the cathode
reactant, the middle term captures the water generated within the
fuel cells 24 and fuel cell stack 22 during operation, and the
third term captures the water leaving fuel cell stack 22 in the
cathode effluent. By using equation (1), holdup observer module 50
can determine the rate of change of overall water holdup in the
fuel cell or in other words the state of hydration. Note that
M.sub.w is sum of water in the membrane (function of .lamda.) and
water in the diffusion media (function of .theta.).
Holdup observer module 50 uses the immediately preceding state of
hydration in conjunction with the immediately preceding rate of
change of the state of hydration to determine the current state of
hydration for fuel cell stack 22. Specifically, the following
equation can be used to determine the current state of
hydration:
dd ##EQU00002##
where:
t.sub.n=current time of interest to make a determination;
t.sub.n-1=previous time at which a determination was made; and
t.sub.n-t.sub.n-1=elapsed time between determinations.
Thus, utilizing the previously determined water uptake in the fuel
cell, the previously determined rate of change of water uptake in
the fuel cell and the time interval between the previous
determinations and the current time of interest, the current state
of hydration can be determined.
Holdup observer module 50, at certain operating conditions of fuel
cell stack 22, can verify the determination of the state of
hydration. Specifically, during periods when the membrane is less
than 100% humidified, holdup observer module 50 can utilize a
measure of the HFR of fuel cell stack 22 to verify the determined
state of hydration. As stated above, the HFR measurement is
directly correlated to the state of hydration of the membrane when
the membrane is less than 100% hydrated. Thus, during periods when
fuel cell stack 22 is operating in a non-flooded state, holdup
observer module 50 can determine the actual state of hydration
based upon the HFR and correct the calculated state of hydration
based upon equation (2). This corrected state of hydration is
utilized in a future determination of the state of hydration at the
next period of interest. Thus, the estimated state of hydration can
be reset or adjusted whenever the conditions lead to partial
humidification of the membrane and the HFR can be measured.
Holdup observer module 50 compares the ascertained state of
hydration to a predetermined standard or range of desired state of
hydration for the operation of fuel cell stack 22. Based on this
comparison, corrective action may be taken to maintain or to
achieve the state of hydration within the predetermined standard or
range, as described in more detail below. Holdup observer module 50
ascertains a target RH set point for the cathode effluent exiting
fuel cell stack 22 based upon the state of hydration and the rate
of change of the state of hydration relative to the predetermined
standard or range. Holdup observer module 50 uses a PI algorithm or
a PID algorithm to determine the target RH set point for the
cathode effluent. The algorithm takes into account how closely the
state of hydration is to the predetermined standard or range. The
algorithm also takes into account the rate at which the state of
hydration is changing and the direction in which the state of
hydration is changing in determining the RH set point.
As the current state of hydration approaches an upper or lower
limit of the desired operational range, the corrective action can
become more drastic, depending upon the rate of change of the state
of hydration and the direction in which the state of hydration is
changing. For example, when the state of hydration is increasing
toward the upper limits, and the rate of change indicates that the
state of hydration is going to continue to increase, a more drastic
corrective action can be taken than when the rate of change of the
state of hydration indicates that the state of hydration is
decreasing. Additionally, the targeted RH set point for the cathode
effluent may also take into account the current operational power
level of fuel cell stack 22. For example, when high current density
operation is occurring, a lower RH set point may be targeted due to
non-equilibrium in humidity between the MEA and the cathode gas in
the cathode flow path. Conversely, during low power operation a
higher RH set point may be targeted due to the membrane humidity
being closer to the relative humidity of the cathode gas in the
flow channels.
The specific algorithm used by holdup observer module 50 will vary
depending upon the design of fuel cell stack 22, and fuel cell
system 20. For example, various fuel cell stacks and/or fuel cell
systems may have components that have different dynamic limitations
and responses in following the load profiles in a typical drive
cycle for the fuel cell system and/or stack. Additionally, the fuel
cell stack and/or fuel cell system may also have different
anticipated drive cycles, depending upon their use, that can also
affect the algorithms chosen. For example, when fuel cell stack 22
is used as a stationary power supply, the power demand may be
relatively constant. In contrast, when the fuel cell stack is used
on a vehicle the power demand may be extremely dynamic and
constantly changing. These differing types of fuel cell stacks may
have different dynamic characteristics to achieve a desired
operation and, accordingly, require different control algorithms to
maintain the state of hydration within a desired interval or
range.
As an alternative to the use of a PI or PID control algorithm,
holdup observer module 50 can use a look-up table module 52 to
determine the appropriate RH set point for the cathode effluent
exiting fuel cell stack 22 based upon the state of hydration, the
rate of change of the state of hydration and the predetermined
standard or range for the state of hydration. Look-up table module
52 contains various tables that provide an appropriate RH set point
for the cathode effluent based upon the operating parameters of
fuel cell stack 22 including the current state of hydration and the
current rate of change of the state of hydration. The data within
the look-up tables are based upon empirical data gathered from
operation of fuel cell stack 22 or an equivalent fuel cell stack.
Thus, the look-up table provides the desired relative humidity set
point for the cathode effluent.
Holdup observer module 50 communicates the target RH set point for
the cathode effluent to an RH control algorithm module 54. Control
algorithm module 54 ascertains appropriate changes/adjustments to
the operating parameters (such as cathode pressure in/out, coolant
temperature in/out, cathode stoichiometry, and cathode inlet RH) of
fuel cell stack 22 to achieve the targeted RH set point for the
cathode effluent. The targeted RH set point and the adjustments to
meet the targeted RH set point are intended to achieve a state of
hydration in fuel cell stack 22 that matches or is within the
predetermined standard or range that provides a desired operation
of fuel cell stack 22. The changes and adjustments also take into
account the power demand placed on fuel cell stack 22.
RH control algorithm module 54 sends the appropriate
changes/adjustments to the operating parameters to a subsystem
controllers module 58. Subsystem controllers module 58 is operable
to adjust the actuators and/or components of fuel cell stack 22 to
impart the appropriate changes/adjustments to the operating
parameters of the fuel cell stack. For example, subsystem
controllers module 58 can control the rate of coolant flowing
through fuel cell stack 22 and the temperature at which the coolant
enters and exits the fuel cell stack. Furthermore, subsystem
controllers module 58 can control the operation of compressor 26 to
allow adjustments to the stoichiometric quantity of cathode
reactant flowing into fuel cell stack 22. Subsystem controllers
module 58 can also control the pressure drop that occurs through
the cathode flow path by adjusting operation of compressor 26 or
various valves or pressure regulators (not shown) in the cathode
flow path. Subsystem controllers module 58 can also control the
operation of WVT device 30 to adjust the relative humidity of the
cathode reactant flowing into fuel cell stack 22. Subsystem
controllers module 58 can also control the operation of an optional
heat exchanger 32 to provide a desired inlet temperature for the
cathode reactant. Accordingly, subsystem controllers module 58 can
adjust various operating parameters of fuel cell stack 22. Control
algorithm module 54 communicates with subsystem controllers module
58 to provide input that the subsystem controllers module 58 uses
to adjust the operating parameters of fuel cell stack 22 to achieve
the targeted RH set point and the desired state of hydration.
Accordingly, in the supervisory control loop of the present
invention, holdup observer module 50 monitors the process
conditions within fuel cell stack 22, determines a state of
hydration (holdup of water) within the stack along with the rate of
change of the state of hydration and determines a target relative
humidity set point for the cathode effluent. As the state of
hydration approaches and/or rises above a desired set point or
upper range value, the target relative humidity set point for the
cathode effluent flowing out of fuel cell stack 22 is reduced. As
the state of hydration approaches and/or falls below a desired set
point or lower range value, the target relative humidity set point
for the cathode effluent flowing out of fuel cell stack 22 is
increased. How fast and how much the relative humidity set point is
modified depends on the extent of process excursions outside the
OCS or, in effect, how much the state of hydration has and is
deviating from its optimal or desired state. Thus, this approach
enables maintenance of the state of hydration within the desired
interval even when the process conditions are outside of the
OCS.
Referring now to FIG. 3, a simplified flow chart of the control
strategy used in the present invention is shown. The control
strategy begins with the starting of the fuel cell stack, as
indicated in block 76. The operating parameters of fuel cell stack
22 are monitored, as indicated in block 78. Holdup observer module
50 uses the operating parameters to determine the current state of
hydration and the current rate of change of the state of hydration
of fuel cell stack 22, as indicated in block 80.
At startup, an initial state of hydration is used as the current
state of hydration. As stated above, the initial state of hydration
can be determined by using a previous state of hydration of the
fuel cell stack upon a previous shutdown, a measure of high
frequency resistance, or operating the fuel cell stack to a flooded
state and using an initial state of hydration of 100%. During
nominal operation of fuel cell stack 22, the determination of the
state of hydration is performed by using equation (2), as described
above. Optionally, the current state of hydration can be corrected
by using a measure of the high frequency resistance, as indicated
in block 82 and discussed above. With the state of hydration
determined, as indicated in block 80a, the rate of change of the
state of hydration is determined, as indicated in block 80b. The
rate of change of the state of hydration is determined using
equation (1), as described above.
The current state of hydration is compared to a predetermined
standard or range, as indicated in block 84. The predetermined
standard or range is chosen to provide a desired and/or optimal
operation of fuel cell stack 22. For example, the state of
hydration can be chosen to provide high current density operation
of fuel cell stack 22. The predetermined standard or range can be
based upon a standardized polarization curve or a performance curve
for the particular fuel cell stack.
Alternatively, the predetermined standard or range can be trimmed
or adjusted based upon the specific previous operating performance
of fuel cell stack 22, as indicated in block 86. The adjustment to
the predetermined standard or range can be done by comparing the
instantaneous voltage and electric current information for fuel
cell stack 22 against a previously achieved best or optimal
operating performance. The previously achieved performance can be
based upon the last few days, weeks, or for an entire history of
operation of the fuel cell stack. By adjusting the predetermined
standard or range to use the best recent operating information as
opposed to a standardized polarization curve, loss of performance
with time or departures of the particular stack performance from
standard can be taken into account. The result is that the
predetermined standard or range can be chosen to provide optimal
performance for the particular capabilities of fuel cell stack 22
and account for changes in those capabilities over time.
Upon comparing the state of hydration to the predetermined standard
or range, a target relative humidity set point for the cathode
effluent flowing out of fuel cell stack 22 is determined, as
indicated in block 88. Holdup observer module 50 can determine the
relative humidity set point using algorithm(s), as shown in block
90, or, alternatively, through the use of look-up table module 52,
as indicated in block 92.
After determining the targeted relative humidity set point for the
cathode effluent, RH control algorithm module 54 determines the
required adjustments to the operating parameters of fuel cell stack
22, as indicated in block 93. Subsystem controllers module 58 then
adjusts the operating parameters, as indicated in block 94,
pursuant to the required adjustments determined by RH control
algorithm module 54.
As fuel cell stack 22 and fuel cell system 20 continue to be
operated, as indicated in decision block 96, the supervisory loop
continues to be active and the control methodology indicated in
blocks 78 through 94 continue to occur. When fuel cell stack 22 is
being shut down, as indicated by decision block 96, controller 46
implements a shutdown procedure, as indicated in block 98. The
shutdown procedures can vary depending upon the particular fuel
cell stack 22 and the use to which fuel cell stack 22 is subjected.
For example, if fuel cell stack 22 is subjected to a freezing
environment, the shutdown procedure may seek to remove a certain
quantity of water or to obtain a certain state of hydration of fuel
cell stack 22 prior to being shut down to avoid damage caused by
freezing conditions. The shutdown procedure may include obtaining
or achieving a desired state of hydration for fuel cell stack 22.
When a specific state of hydration is achieved, this information
can be used in subsequent operation of fuel cell stack 22 as the
initial state of hydration upon startup of operation, as discussed
above. After performing the shutdown procedures, operation of fuel
cell stack 22 ends, as indicated in block 100.
Accordingly, the operation of a fuel cell stack utilizing the
methods of the present invention allows high current density
operation and actively manages excursions outside the OCS. This in
turn enables operation at higher efficiency and also helps low
power instability issues associated with fuel cell flooding.
Moreover, this approach minimizes the impact of process condition
excursions on the durability of the fuel cells within fuel cell
stack 22. Additionally, this control approach minimizes the cycling
between flooded and dry conditions even during the transients
associated with a fuel cell stack utilized in a vehicle drive
cycle. Thus, the present control strategy advantageously considers
the dynamic impact of process excursions on the fuel cell
performance and actively managing these process excursions. The
actively managing of these process excursions provides superior
performance than previously utilized steady state oriented control
approaches.
While the present invention has been shown and described with
reference to a specific fuel cell system 20 and supervisory control
loop, it should be appreciated that variations can be made without
departing from the spirit and scope of the present invention. For
example, the mechanization of fuel cell system 20 can vary from
that shown. Fuel cell system 20 may utilize devices other than WVT
device 30 and/or heat exchanger 32 to provide those
functionalities. Furthermore, while the present invention has been
disclosed as providing a targeted relative humidity set point for
the cathode effluent exiting the fuel cell stack 22, it should be
appreciated other parameters, such as cathode in/out pressure,
cathode stoichiometry, cathode inlet RH and coolant temperature
in/out, can have a targeted set point to achieve a desired
operation of fuel cell stack 22 while the remaining parameters are
adjusted to meet the targeted set point. In addition, the holdup
model as described earlier could additionally contain terms for
anode inlet and anode outlet. Thus, the description of the
invention is merely exemplary in nature and variations that do not
depart from the gist of the invention are intended to be within the
scope of the invention. Such variations are not to be regarded as a
departure from the spirit and scope of the invention.
* * * * *